The wear which occurs on the internal surface of a gun barrel as a projectile passes through and from the barrel is well known. Such wear and erosion of the surface of the barrel is due in part to the abrasion of the surface of the projectile against the internal wear surface of the gun barrel. In addition, the propellant and propellant gases may also cause abrasion, wear, chemical erosion and occasionally melting. Erosion by melting may be aggravated by the so called "blow by" phenomenon in which extremely high velocity gas passes between portions of the projectile and the wall of the barrel as the projectile is accelerated along the length of the barrel and projected from the muzzle.
While some of these problems can be overcome by the use of exotic metal alloys the cost of construction of gun barrels of such alloys makes such construction too costly. Almost all production gun barrels are made from a low alloy wrought steel having less than 8% alloying elements.
Also attempts have been made to improve the wear resistance and projected useful life of gun barrels by plating with chromium. Other barrels have a short cobalt-based liner at the breech end to reduce erosion of barrel metal. The liner is not metallurgically bonded to the barrel steel.
Where propellants having higher flame temperatures are employed or where very high energy or high velocity projectiles are fired in rapid succession with long bursts from the gun barrels, the current gun barrels do not have acceptable life due to excessive wear at the internal surfaces and due to related reasons.
The mode of failure of structures designed for specific end uses such as gun barrels can be determined by basic mechanisms. One such mechanism is the rate at which heat can be transferred from a surface which receives the heat through the structure to a surface which can dissipate the heat. For example, in a gun barrel the heat is received by the barrel at the barrel interior due to the burning and heat of burning of the propellant material. In addition, frictional force of the projectile moving along and against the surface of the interior of the barrel can generate heat at the immediate surface contacted by the projectile. Where the amount of heat which can be removed from the barrel through normal conduction mechanism is limited, this places a limit also on the application which can be made of the gun. If temperatures become excessive, the gun barrel may fail either locally at the inner surface of the gun barrel by localized melting or metal deformation at high temperature or the physical properties of the overall structure of the barrel may deteriorate resulting in a rupture.
Another mode of failure is the simple mechanical failure to contain the mechanical forces which are applied on the gun barrel. For example, as a propellant is ignited and burns it generates not only heat but also very high pressure and this pressure must be mechanically contained by the barrel. Also, where the projectile leaves its cartridge and starts down the barrel the rifling on the barrel mechanically applies a torsional force to the projectile to give it spin necessary to aid it in its accurate flight to a destination or target. Where the mechanical force needed to initiate rotation of the projectile is excessive, mechanical failure of the barrel can occur at the location adjacent to the chamber where the barrel rifling starts.
Regarding the heat generated at the bore of a gun barrel this heat can build up very rapidly in spite of the fact that the heat can be transferred through the wall of the barrel to the barrel exterior because of the higher rate at which heat can be produced at the bore compared to the rate at which the produced heat can be carried by heat conduction through the thickness of the barrel wall. For a barrel wall of lower conductivity, when long bursts of firing occur, or when the heat produced by the gases are relatively high, this heat production is concentrated at the bore surface and cannot be conducted from the bore rapidly enough because of the limitations in the conductivity of heat through the material of the barrel wall.
There is a heat sink effect in the thickness of the barrel but this heat sink is available only until the temperature of the barrel itself is raised by production of heat within the bore in excess of the quantity of heat which can be conducted through the wall thickness based on the characteristics of the material of the wall itself.
In fact the combined barrel and propellant must be treated as a system because all the elements of the gun must be kept in balance. Any one element which is out of balance with the others can cause failure. For example, if the propellant generates excessive pressure or temperature or is used in excessive amount, this alone could disrupt the balance between the several components of the system and lead to excessive heat and thermal degradation of the barrel or bore surface.
It is recognized in the industry that if guns are designed to fire projectiles at significantly higher rates and velocities and at higher energies, higher performance gun barrels will be needed.